Why Do Larger Nuts Always Rise to the Top?

By Kevin Ritchart

A new experiment has revealed why the largest pieces in some mixtures tend to rise to the top.

The phenomenon is known as the Brazil nut effect, named for the large type of nut that ends up at the top of packages of mixed nuts.

This effect also occurs in cereal boxes, where the larger pieces tend to collect on top. The Brazil nut effect can even be applied to asteroids, where larger rocks cluster on the outside.

How Does It Work?

Knowing how the Brazil nut effect works could be useful in manufacturing. If engineers know why particles separate by size, they would be able to build machines that are better equipped to avoid the issue, potentially leading to a more uniform mixture of ingredients in food processing.

Parmesh Gajjar, an imaging scientist at the University of Manchester in England, recently led a team of researchers in studying the Brazil nut effect. The team’s findings were published in the April 19 issue of Scientific Reports.

The biggest challenge in studying the Brazil nut effect is that it’s difficult to track how individual objects move in the middle of a mixture. Gajjar’s team dealt with this challenge using X-rays.

The team took scans that tracked the motion of individual peanuts and Brazil nuts in a box that was being shaken. This allowed the researchers to create 3-D video renderings of the Brazil nut effect in action.

Shaking Things Up

When the shaking began, the oval-shaped Brazil nuts mostly laid sideways in the box. But as the box was shaken back and forth for a longer period of time, the nuts began bumping into each other.

These collisions caused some of the Brazil nuts to move from lying sideways to being positioned vertically. And it turns out that the vertical orientation was the key to the Brazil nuts being able to rise through the pile and toward the top of the box.

Once the Brazil nuts were vertical, there were open spaces alongside them for the smaller nuts to tumble downward. As the smaller nuts moved down, the Brazil nuts rose to the top of the box.